Various obstacle detection systems have been proposed to warn drivers of the presence of an object in the path of a moveable vehicle. Typically, such warning systems provide a suitable audio and/or visible signal when an object is detected in the path of the vehicle.
In consideration of the importance of the operability of such object detection systems, it is often desirable for such systems to include self testing capabilities. For example, U.S. Pat. No. 5,160,927, of common Assignee and incorporated herein by reference, describes an object detection system with a self test feature wherein object detection signals transmitted by the transmitter are received directly by the receiver for a self test check. Another object detection system with a self test feature is described in U.S. Pat. No. 4,910,512 in which signals provided by a transmitter are reflected by acoustic shunts or acoustic reflectors for reception by a receiver for self testing purposes. Furthermore, U.S. Pat. No. 4,561,064 to Bruggen et al. describes an object detection system with a self test feature in which a fixed reflector located on a vehicle reflects signals from the transmitter to the receiver to evaluate the system.
Another obstacle detection system which utilizes self testing features is described in U.S. Pat. No. 5,235,315, of common Assignee and incorporated herein by reference, in which a controller is provided for adjusting the gain and signal threshold of a receiver so that the receiver initially is able to detect reflections from irregularities in the ground surface. Once this receiver receives a transmitted signal reflected from the ground surface, an indication is provided that the system is in a ready or operable state.
U.S. Pat. No. 5,432,516, which is also assigned the Assignee of the present invention and hereby incorporated herein by reference, articulately discloses a radar obstacle detection system that includes a self-test to detect system failures. The system uses a dedicated auxiliary-antenna which radiates a delayed test signal to the main antenna of the system. The delayed test signal received by the main antenna is compared against an expected signal to detect operational faults in the system.
Although the foregoing systems perform various tests to confirm system integrity, there is a need for an obstacle detection system having a built-in-test which does not require additional reflecting elements and/or specific positioning of the transmitter and receiver in order to avoid interference caused by preexisting structures on the vehicle, or rely upon signals reflected from outside the systems such as a ground surface.